one of the first things i notice on threads like these is that a lot of people actually like when this happens. they say the calyxes "stack".
when foxtailing happens to me it is usually nothing more than a stretching of the bud, actually resulting in less density and more sticks. i really hate it actually.
i tend to see it in strains that are particularly on the sativa side and have "waves" of pistils, meaning that the plant seems done but it's shooting out new pistils everywhere. i have decided that in this case it is a mistake to let the plant continue to ripen because it just causes stretching of the "bud tips", and sometimes the bud can get overdone.
i also notice it when the plants have been subjected to a fair amount of moisture and heat stress during the flower cycle. i almost never notice it on strains that are naturally dense.
a comment above linked it to recovery, meaning that the plant was stressed, it recovered, and the foxtails are a reflection of whatever processes took place. i never noticed this exact link myself but stress does all sorts of wacky stuff to plants.